People who were overweight or mildly obese -- weighing between 146 and 203 pounds for a 5'4" person -- had a 5 to 6 percent lower risk of dying compared to those who were at a healthy weight, between 108 and 145 pounds. Those who were extremely obese had nearly a 30 percent greater risk of dying...
"What we found suggests that over a broad range of body mass index levels, there's not much variation in mortality,"
... added that the new study had flaws in its methodology.
For example, older folks in the lower end of the healthy weight spectrum may have been more likely to have been former smokers with health problems that kept them slim.
Those researchers found a clear relationship between increasing body mass index and death rates with a 13 percent greater death risk in moderately overweight individuals, which rose to a nearly 90 percent greater risk in those who were obese.
If I can add my 2 cents here, I am not sure that there is a clear cause and effect (causality). It is rare that I hear anyone say why being overweight would increase your chance of death. Instead there seems to be a casual association with other health problems.
It is possible that things that cause people to be overweight might also cause them to have a higher risk of death. This could include lack of exercise, metabolic syndrome, consumption of saturated fat and sugar, or just not eating enough fruits and vegetables. For example, one study showed that doing exercise can offset the danger of being obese. For years I have seen people claim that heavy people who exercise have lower mortality than thin people who don't.
One problem with this theory is that heavy people aren't very motivated to exercise.